Exposure of the female genital tract to HIV-1-infected semen is the primary route of heterosexual HIV transmission. Gaining a detailed understanding of this process is critical for developing vaccines and prophylactic topical microbicides. In this proposal, we investigate the effect of small membrane vesicles in semen, seminal exosomes, on HIV transmission and the induction of cervicovaginal immune responses. Semen has the highest concentration of exosomes among all body fluids studied, but relatively little is known about the biology of these particles. Our preliminary studies show that seminal exosomes associate with cell-free HIV-1 virions in seminal plasma and enhance HIV-1 infectivity. Moreover, seminal exosomes rapidly enter vaginal Langerhans cells (LCs) upon contact. Building on these findings, we will directly study three modes of potential infection enhancement by seminal exosomes in the female genital mucosa, using both a polarized and a non-polarized ex vivo cervicovaginal explant model: productive HIV infection of intraepithelial target cells, HIV penetration into the cervicovaginal epithelium, and HIV entry into vaginal LCs. Given the reported immunosuppressive properties of seminal exosomes, we will also investigate whether LC-mediated adaptive T-cell responses to alloantigens or recall antigens are blunted when seminal exosomes invade the LCs. If seminal exosomes facilitate sexual HIV transmission, microbicidal prevention strategies could include a blockade of virion-exosome interactions or an inhibition of exosome penetration into the mucosa. If processing of seminal exosomes by vaginal LCs tolerizes the genital immune system to foreign antigens, mucosal vaccination strategies could benefit from controlling this effect. PUBLIC HEALTH RELEVANCE: Worldwide, most HIV infections occur through the linings of the genital or intestinal tract following sexual contact with HIV-1-containing semen. Information about the factors in human semen that influence the ability of HIV-1 to infect target cells in the mucosa is therefore needed to help in devising medical strategies for prevention, such as vaccines or locally delivered antiviral drugs. In this proposal, we plan to study whether small but abundant membrane vesicles in semen, termed exosomes, have an effect on HIV infection and the induction of immune responses in the vagina. If exosomes in semen facilitate sexual HIV transmission or blunt the immune response to HIV infection and vaccination, an exact definition of these pathogenic pathways will inform prevention strategies against sexual HIV transmission.